Coal-fired utility boilers for electricity generation is nearly a 70 year old commercially-accepted technology and is expected to continue for many years. However, it is imperative that we make it an environmentally-responsible technology. Since the Clean Air Act legislation in 1970, significant progress has been made by power generation industries to reduce emissions of NO, SO.sub.2, and particulates. Attention is now focused on the mitigation of carbon dioxide emissions that is believed to cause global climate change. Energy Information Administration's International Energy Outlook estimates that nearly 2.5 billion metric tons of CO.sub.2 are emitted due to coal use and is projected to grow to nearly 4 billion metric tons by 2020. The CO.sub.2 emissions from coal use are primarily from electricity generating conventional coal-fired power plants that customarily use air as the oxidant.
There is currently no enabling technology for mitigating CO.sub.2 emissions to our environment. Potential strategies include the capture of CO.sub.2 emitted from coal-fired stationary combustion sources and either sequester it or use it as a feedstock for products of commerce. World-wide CO.sub.2 emissions from all fossil burning sources amount to nearly 6 billion metric tons and is always emitted as dilute streams. Capturing the CO.sub.2 from stationary combustion sources as dilute streams containing nearly 75% nitrogen, concentrating the CO.sub.2, and then reacting it to make economically-attractive products of commerce is a formidable challenge. Ocean or geologic sequestration is not always viable for the simple reasons of transportability and logistics.
An alternative to the dilute stream capture and sequestration option is to design and/or modify power systems configuration to make such combustion systems inherently-low CO.sub.2 emitting technologies. One such concept is to use a mixture of oxygen and carbon dioxide as the oxidant instead of air in stationary combustion processes. The result is a CO.sub.2 -enriched flue gas stream that contains no nitrogen and requires much smaller process equipment to capture, concentrate, and then sequester. The flue composition is primarily CO.sub.2 and water. A portion of the flue gas may also be recycled following hot gas clean-up operation for NOx, and SO.sub.2, and particulate removal. In addition, O.sub.2 --CO.sub.2 combustion mode reduces furnace volume and therefore capital for new plants, reduces load to the flue gas desulfurization and NO.sub.x reduction units, requires lower power for auxiliary load draws for primary and secondary combustion fans, lowers duty to the Heat Recovery Steam Generator unit, and provides heat rate improvement and high flame temperature. This improves combustion efficiency, and the efficiency improvement translates into lower CO.sub.2 emissions.
Oxygen derived by conventional cryogenic technology is not a viable option for oxygen-enriched combustion of coal-fired boilers. This is because cryogenic technology requires extremely low temperature (below -290.degree. F.) where air becomes a liquid and oxygen is separated by distillation. A conventional power plant heats steam to over 1000.degree. F. for driving a turbine. With the availability of high temperature heat exchanger material, future power plants would be operated at much higher temperatures. This level of temperature spread, cryogenic on one side and very high temperature required for coal combustion on the other side, is not acceptable to electricity producing industries. A technology that would use the power plant's heat sources to thermally energize an air separation process and would produce low-cost oxygen, would enhance the acceptance of combustion modification by the electric utility industry sector, especially to comply with voluntary or involuntary carbon emission regulations. Availability of low-cost oxygen would also make oxygen-blown Integrated Gasification Combined Cycle an economic power producing choice.
Ceramic membranes are being developed that will selectively separate oxygen from air at high temperature and pressure. These are solid electrolytes that allow ionization of oxygen on one surface followed by conduction of the oxygen anion to the other surface where molecular oxygen is reformed and released electrons are transported counter currently to ionize incoming oxygen molecules in the air. For example, U.S. Pat. Nos. 5,580,497, 5,639,437, and 5,573,737 (the disclosures of which are incorporated by reference) disclose oxygen anion and electron conducting ceramic membranes having requisite stability at several operation-regeneration process cycles. Generically these membranes can be represented by the formulae ABO.sub.3 where appropriate substitution of the cations, A and B. creates ion vacancies that serve as pathways for the oxygen anions. The dense ceramic membranes disclosed in the '497, '437, and '737 patents provide the technology for oxygen-enriched combustion in coal-fired boilers as a mechanism for CO.sub.2 remediation and handling.
The gas-impervious mixed metal oxide materials useful in ceramic membranes of this invention include any single phase and/or multi-phase, dense phase, intimate mixture of materials which together form a solid material having electron conductivity and oxygen ion conductivity. As used herein, the term "gas-impervious" means "substantially gas-impervious or gas-tight " in that the material does not permit a substantial amount of oxygen-containing gas stream or another, organic-containing, gas stream to pass through the solid mixed metal oxide materials of the ceramic membranes as a gas (i.e., the solid mixed metal oxide materials are non-porous, rather than porous, with respect to the relevant gases).
In particular, it has been found that mixed metal oxides having a perovskite structure (at operating temperatures) can have useful levels of oxygen ion conductivity. Materials known as "perovskites" are a class of materials which have an X-ray identifiable crystalline structure based upon the structure of the mineral perovskite, CaTiO.sub.3. In its idealized form, the perovskite structure has a cubic lattice in which a unit cell contains metal ions at the corners of the cell, another metal ion in its center and oxygen ions at the midpoints of each cube edge. This cubic lattice is identified as an ABO.sub.3 -type structure where A and B represent metal ions.
In the idealized form of perovskite structures, generally, it is required that the sum of the valences of A ions and B ions equal 6, as in the model perovskite mineral, CaTiO.sub.3. A relationship between radii of ions in an ABO.sub.3 -type structure containing two metal ions is expressed by the formula: EQU r.sub.A +r.sub.X =.tau..times.(r.sub.B +r.sub.X).times.(2.sup.0.5)
where r.sub.A, r.sub.B, and r.sub.x are, respectively, radii of A ions, B ions and oxygen ions and .tau. is a factor having values in a range from about 0.7 to about 1.0. Typically, compounds with the perovskite structure have A ions with radii of between about 1.0 to about 1.4 Angstrom and B ions with radii of between about 0.45 to about 0.75 Angstrom. It appears, generally, that when mixed metal oxides of the perovskite structure contain A ions with radii approaching the low end of their range for a specific B ion as given by the formula above, then conductivity of oxygen ions generally increases. This trend toward increased conductivity of oxygen ions may be limited, however, by lowered stability of the perovskite structures at operating temperatures for A ions with radii approaching the low end of their range for a specific B ion.
A variety of selected chemical elements and compounds of selected elements, such as acetates, carbonates, chlorides, oxides, nitrates, etc., may be used to form perovskites useful in the present invention. Generally, any combination of metallic elements satisfying requirements of a perovskite may be used. Examples of useful chemical elements include beryllium (Be, Atomic No. 4), magnesium (Mg, Atomic No. 12), calcium (Ca, Atomic No. 20), scandium (Sc, Atomic No. 21), titanium (Ti, Atomic No. 22), vanadium (V, Atomic No. 23), chromium (Cr, Atomic No. 24), manganese (Mn, Atomic No. 25), iron (Fe, Atomic No. 26), cobalt (Co, Atomic No. 27), nickel (Ni, Atomic No. 28), copper (Cu, Atomic No. 29), zinc (Zn, Atomic No. 30), gallium (Ga, Atomic No. 31), strontium (Sr, Atomic No. 38), yttrium (Y, Atomic No. 39), zirconium (Zr, Atomic No. 40), niobium (Nb, Atomic No. 41), barium (Ba, Atomic No. 56), lanthanum (La, Atomic No. 57), cerium (Ce, Atomic No. 58), praseodymium (Pr, Atomic No. 59), neodymium (Nd, Atomic No. 60), promethium (Pm, Atomic No. 61), samarium (Sm, Atomic No. 62), europium (Eu, Atomic No. 63), gadolinium (Gd, Atomic No. 64), terbium (Tb, Atomic No. 65), dysprosium (Dy, Atomic No. 66), holmium (Ho, Atomic No. 67), erbium (Er, Atomic No. 68), thulium (Tm, Atomic No. 69), ytterbium (Yb, Atomic No. 70), lutetium (Lu, Atomic No. 71), and mixtures thereof.
Preferred A metal ions in the ABO.sub.3 -type structure materials useful in the present invention include ions of the lanthanide series of elements in the Periodic Table of Elements (Atomic Nos. 57 to 71 inclusive), yttrium ions (Atomic No. 39), and ions of the Group IIA elements in the Periodic Table of Elements, particularly magnesium ions (Atomic No. 12), calcium ions (Atomic No. 20), strontium ions (Atomic No. 38) and barium ions (Atomic No. 56).
Preferred B metal ions-in the ABO.sub.3 -type structure materials useful in the present invention include ions of the first row of transition elements in the Periodic Table of Elements, i.e., scandium ions (Atomic No. 21), titanium ions (Atomic No. 22), vanadium ions (Atomic No. 23), chromium ions (Atomic No. 24), manganese ions (Atomic No. 25), iron ions (Atomic No. 26), cobalt ions (Atomic No. 27), nickel ions (Atomic No. 28), copper ions (Atomic No. 29), and zinc ions (Atomic No. 30). Among these ions, cobalt ions and iron ions are more preferred.
A wide variety of multiple cation substitutions on both the A and B sites are stable in the perovskite structure. Likewise, a variety of more complex perovskite compounds containing a mixture of A metal ions and B metal ions are useful in this invention. Preferred for use in the present invention are materials having a perovskite structure containing metal ions of more than two elements (in addition to oxygen). Crystal structure of these mixed metal oxide compounds need not be pure perovskite it could be a mixed-crystalline phase material and perovskite could be one of those crystalline phases.
Examples of mixed metal oxides which are useful as solid oxygen ion-conductive ceramics in the present invention include lanthanum-strontium-cobaltite, lanthanum-calcium-cobalite, lanthanum-strontium-ferrie, strontium-ferrite, strontium-cobaltite, gadolinium-strontium-cobaltite, etc., and mixtures thereof. Specific examples included are La.sub.a Sr.sub.b CoO.sub.3, La.sub.a Ca.sub.b CoO.sub.3, La.sub.a Sr.sub.b FeO.sub.3, SrCo.sub.a Fe.sub.b O.sub.3, Gd.sub.a Sr.sub.b CoO.sub.3, etc., were a and b are numbers, the sum of which is in a range from about 1 to about 1.5. Molar ratios between the respective metals represented by the ratio a:b may range widely, e.g., 4:1, 3:1, 1:1, 1:4, 1:3, etc. Particularly preferred are materials represented by SrCo.sub.0.5 FeO.sub.x, SrCo,.sub.0.8 Fe.sub.0.2 O.sub.x, and La.sub.0.2 Sr.sub.0.8 Co.sub.0.4 Fe.sub.0.6 O.sub.x.